Baltimore, Md. — A man was shot and killed while pumping gas at a suburban Virginia gas station Wednesday night, and police were investigating to determine whether the sniper that has terrorized the Washington area had struck again.

Virginia state police said two males were seen driving away in a white vehicle after the shooting at the station in Prince William County, near Manassas, 40 kilometres west of Washington.

Prince William County police spokeswoman Sgt. Kim Chinn said one man was killed. Chinn said she had few details to release. She said police did not know where the shots came from or how many shots were fired.

Police had blocked off several streets around the gas station, and they were interviewing people at the scene.

"We are still very preliminarily beginning this investigation," Sgt. Chinn said. "We have been in contact with the officials from Maryland and the task force there, so we are sharing any information we have."

Meanwhile, a tarot card with the words "Dear policeman, I am God" emerged as a potential clue.

The card was found near a shell casing outside a middle school in Bowie, where a 13-year-old boy was critically wounded by the gunman Monday, a source familiar with the investigation said on condition of anonymity.

Authorities said the shell was a .223-calibre, the same kind of bullet used to kill six people and wound another in Washington and its Maryland and Virginia suburbs in the last week. The casing is believed to be the first one recovered since the slayings began.

Michael Bouchard, an agent with the Bureau of Alcohol, Tobacco and Firearms, would not say whether authorities had linked the casing to the attacks.

Montgomery County Police Chief Charles Moose also wouldn't comment when asked about the tarot card, and angrily suggested unapproved information had been leaked.

"I need to make sure I don't do anything to hinder our ability to bring this person or these people into custody," Chief Moose said.

The message left on the tarot card called the Death card was first reported by WUSA-TV and then by the Washington Post. Police sources told the newspaper the items were found 140 metres from the school in a wooded area on matted grass, suggesting the gunman had lain in wait.

Tarot cards, used mainly for fortune-telling, are believed to have been introduced into western Europe by Gypsies in the 15th century. Many tarot enthusiasts say the Death card usually does not connote physical death, but instead portrays a symbolic change or transformation.

Crime experts, while noting that the link between the card and the sniper remained unconfirmed, recalled other serial killers who left "calling cards."

One of the most notorious was David Berkowitz, who killed six people in New York in 1976-77. He wrote a letter to newspaper columnist Jimmy Breslin and left a note addressed to a police detective that said: "I am a monster. I am the 'Son of Sam."'

Robert Ressler, a former FBI profiler, interviewed Mr. Berkowitz after his arrest.

"He said this was a stimulating thing for him to see the letters in the paper," Mr. Ressler said. "Even though he's the only one who knows, notoriety becomes very satisfying to an inadequate loser. It's a way of imposing power and control over society."

Mr. Ressler recalled one previous case, in 1970, in which the murderer of a wealthy ophthalmologist and his family near Santa Cruz, Calif., left a tarot card. The killer was captured and sentenced to life in prison.

The motive for the seemingly random attacks remains unknown. Nearly 200 investigators are working their way through some 8,000 tips. One tip sent them on a fruitless search of woods behind a school in Prince George's County, but nothing was found.

All the victims have been felled by a single bullet. Investigators say the sniper, or snipers, fired from a distance with a high-powered hunting or military-style rifle.

The wounded boy, whom police have not identified, remained in critical but stable condition Wednesday. Ballistics tests found that the bullet that struck him was of the same calibre as those that killed some of the others and wounded a woman in Virginia. That woman was released from hospital Tuesday.